Plastic films have been widely used as packing materials, electronic materials such as for video tapes, and optical materials such as photo films. These plastic films are formed from a resin which is in a molten state or in a state of a solution. In many cases, these films are used as stretched films being stretched monoaxially or biaxially. A plastic is a high molecular material having a long molecular chain which is oriented upon being stretched. Therefore, a plastic film exhibits improved resistance against bending, bend recovering property, toughness, etc.
In recent years, plastic films have been widely used in the optical field such as of liquid crystal display devices, and it has been desired to further improve their properties. That is, to meet higher performances required for the films, the resin that becomes a material therefor has been produced having an ever complex molecular structure, such as being modified and blended, pushing up the cost. Further, in order for the film to exhibit its functions to a more than enough degree, it has been demanded not only that the film exhibits homogeneous properties but also that the film has a uniform thickness over the whole surface thereof.
As the films comprising a thermoplastic resin, there can be exemplified an unstretched film obtained by extruding a resin heated and melted in an extruder onto the surface of a casting roll through a slit of a T-die and winding it like a coil, a monoaxially stretched film obtained by extruding the resin onto the surface of the casting roll and stretching it in the lengthwise direction only, and a biaxially stretched film obtained by stretching the extruded resin in the lengthwise direction and in the direction of width. Any of these films ejected from the T-die and extruded onto the casting roll solidifies in such a manner that both end portions thereof become unavoidably thicker than the central portion thereof due to the properties of a highly viscous molten resin. Therefore, both end portions of the films are cut and removed to meet the allowable thickness in the direction of width that is required.
The yield of the resin material increases with a decrease in the width by which the film must be cut and removed at both end portions of the film where the thickness deviation is large. From the standpoint of producing the films, therefore, it is an important problem to decrease the width that must be removed.
For example, the following patent document 1 discloses a method of producing unstretched films comprising thermoplastic resins of many kinds in small amounts maintaining high yields. That is, in forming the unstretched film that is ejected from the T-die and is extruded onto the casting roll, there are coextruded only those portions that are solidified in such a manner that both end portions thereof become unavoidably thicker than the central portion due to the properties of the highly viscous molten resin, and those portions only are cut and removed.
As described above, while the stretched films are now exhibiting ever sophisticated functions, it has been demanded to further uniformalize the thickness profile of the films. In forming the unstretched film, a deviation in the thickness at both end portions of the film becomes a problem occurring in such relatively short periods of time as in a step where the resin falls from an ejection port of a T-die of an extruder onto the casting roll and a step where the resin solidifies on the surface of the casting roll. In the case of a stretched film, further, there remains a problem in that deformation due to stretching through a plurality of re-heating rolls and between the stretching rolls, greatly affects the deviation in the final thickness at both end portions.
The prior art information related to the present application is disclosed in the following document.    Patent document 1: JP-A-2005-246607